The invention relates to a secondary assembly drive of an internal combustion engine and to a method for operating this drive. The secondary assembly drive comprises:                a first drive that can be rotationally connected to a crankshaft of the internal combustion engine,        an electric machine that can be operated selectively as a generator or as a motor and a second drive wheel that can be rotationally connected to the electric machine,        an air-conditioning system compressor and a third drive wheel that can be rotationally connected to the air-conditioning system compressor,        an endlessly rotating traction mechanism that wraps around the drive wheels,        and a controllable coupling for the drive disconnection of the electric machine and the air-conditioning system compressor from the crankshaft when necessary.        
A belt drive according to the class for secondary assembly drives of internal combustion engines emerges from DE 10 2008 023 834 A1. Switchable couplings are provided both between the crankshaft and the associated crankshaft belt pulley and also between each of the ancillary drives and the associated drive pulley. These form the basis for a series of additional functionalities for the ancillary drive, in that the ancillary drives are driven selectively by the crankshaft or by the electric machine formed as a starter generator or also not at all. Thus, the starter generator allows, as additional functionalities, not only a starting of the belt of the internal combustion engine, but also air conditioning for the vehicle when the internal combustion engine is stopped. When providing air conditioning when the vehicle is parked, the (stopped) crankshaft is separated from the drive of the belt drive and the air-conditioning system compressor is driven electromotively. An analogous operation is applicable here for the coolant pump.
When the internal combustion engine is running, the crankshaft-side coupling is opened so that the entire ancillary drive is separated from the drive of the crankshaft and stopped accordingly.